This invention relates to the field of image sensing, and in particular to the automated measurement of incident illumination of a sensor in a video projection display.
In a projection video display, geometrical raster distortions result from the physical placement of the cathode ray display tubes. Such raster distortions are exacerbated by the use of cathode ray tubes with curved, concave display surfaces and the inherent magnification in the optical projection path. The projected image is composed of three scanning rasters which are required to be in register one with the other on a viewing screen. The precise overlay of the three projected images requires the adjustment of multiple waveforms to compensate for geometrical distortion and facilitate the superimposition of the three projected images. However, manual alignment of multiple waveforms is labor intensive during manufacturing, and without the use of sophisticated test equipment may preclude setup at a user location. Thus an automated convergence system is disclosed which simplifies manufacturing alignment and facilitates user location adjustment. An automated alignment system may employ raster edge measurement at peripheral display screen locations to determine raster size and convergence. However, at peripheral screen locations, sensor illumination levels may vary widely causing erratic or inconsistent detection of sensor signals thus a detector is required which obviates detection inconsistencies.
A projection display apparatus comprises a display device forming an image for projection to illuminate a display screen. A current source generates a current. A current sink is adjacent to the display screen and is coupled to the current source for receiving the current. A detector is coupled to the current source and the current sink for receiving the current and detecting a presence of the received current. Wherein a first condition, absent illumination by the projected image on the current sink, the current from the source divides and is coupled to the detector in accordance with a first ratio, the detector assuming a first state indicative of the absence of illumination, and in a second condition in accordance with an intensity of illumination by the projected image on the current sink, the current from the current source is received by the current sink in accordance with a second ratio and the detector, absent the received current, assumes a second state indicative of illumination on the current sink.